What Is the Triboelectric Effect and What Are Its Applications?

Imagine for a moment that your gloves or shoes were a source of energy, powering wearables, sensors, or your smartwatch. That's the promise of triboelectricity and the new TENGs, i.e., triboelectric nanogenerators. Renewable energies such as wind or photovoltaics are becoming increasingly important in the energy mix. Still, other more unknown energies could also contribute in their way, some on a large scale such as tidal or wave energy and others in the domestic environment such as triboelectricity. The triboelectric effect is nothing more than what we know as static electricity. The difference is that the latest research is discovering new ways of harnessing it. When integrated into textile fibers or even floors, it can turn the most unsuspected objects into clean electricity sources.  

Applications of the triboelectric effect

• Floors with a triboelectric layer connected to IoT devices
• Sensors in smart clothing to measure vital signs
• Shoes capable of monitoring physical exercise
• Autonomous sensors as forest fire detectors
• Tactile sensors
• Dust and particle sensors
• LED power supply

How do triboelectric nanogenerators work?

Usually, the triboelectric effect occurs with friction between two surfaces, where one gives up electrons to the other. This is what sometimes happens when you run a comb through your hair or take off a sweater. Triboelectric nanogenerators, first proposed theoretically in the early 2010s, exploit this principle to power miniature sensors. A basic TENG contains four layers. The top one is responsible for releasing electrons, the middle one traps them, and the bottom one collects them. All three, in turn, are covered by another layer that acts as a battery to store the current generated, which must be converted from alternating current (AC) to direct current (DC).

Specific materials such as lipids or nylon are used to optimize the layer's functioning responsible for releasing electrons from a TENG. However, what mostly determines its efficiency is its shape. In other words, if on a large scale, roughness contributes to enhancing friction and electron release, this principle is transferred to the nanoscale. The latest generation TENGs use microstructures to generate more electricity, as these micro-reliefs exponentially multiply the contact surface. Subsequently, the relief of a triboelectric layer receives various treatments such as exposure to a negatively ionized air stream or a plasma treatment that further increases its electrical capacity.

Friction is not the only way to exploit the triboelectric potential. For example, a triboelectric membrane can generate a current thanks to the impact of raindrops.

Triboelectricity in sensors

The currents generated by such devices are very weak, mostly measured in milliwatts. However, this amount of energy is enough to power an LED or devices integrated into smart clothing such as shoes or T-shirts. Furthermore, a triboelectric system does not necessarily have to be used to generate electricity to power other devices. The fact that it is activated by friction or mechanical pressure makes it an ideal candidate for sensors. For example, artificial skin can register touch thanks to a triboelectric layer. Also, they generate current when subjected to torsion or stretching, making it possible to monitor joint movements. 

TENGs are complexly designed devices, but they have infinite applications for use in wearables and integration into autonomous sensors that can operate in contexts as disparate as the home, a forest, or the sea. 

Source: Nature, Frontiers in Mechanical Engineering

Imagen: Georgia Tech